A common feature of most multicellular organisms is the progressive and irreversible physiological decline that characterizes senescence. Although genetic and environmental factors can influence the aging process, the molecular basis of senescence remains unknown. Postulated mechanisms include cumulative damage to DNA leading to genomic instability, epigenetic alterations that lead to altered gene expression patterns, telomere shortening in replicative cells, oxidative damage to critical macromolecules and nonenzymatic glycation of long-lived proteins (S. M. Jazwinski, Science 273:54, 1996; G. M. Martin, et al., Nature Gen. 13:25, 1996; F. B. Johnson, et al., Cell 96:291, 1996; K. B. Beckman and B. N. Ames, Physiol. Revs. 78:547, 1998). Factors which contribute to the difficulty of elucidating mechanisms and testing interventions include the complexity of organismal senescence and the lack of molecular markers of biological age (biomarkers). Aging is complex in that underlying mechanisms in tissues with limited regenerative capacities (e.g., skeletal and cardiac muscle, brain), which are composed mainly of postmitotic (non-dividing) cells, may differ markedly from those operative in proliferative tissues. Accordingly, approaches which provide a global assessment of senescence in specific tissues would greatly increase understanding of the aging process and the possibility of pharmaceutical, genetic or nutritional intervention.
Genetic manipulation of the aging process in multicellular organisms has been achieved in Drosophila, through the over-expression of catalase and Cu/Zn superoxide dismutase (W. C. Orr and R. S. Sohal, Science 263:1128, 1994; T. L. Parkes, et al., Nat. Genet. 19:171, 1998), in the nematode C. elegans, through alterations in the insulin receptor signaling pathway (S. Ogg, et al., Nature 389:994, 1997; S. Paradis and G. Ruvkun, Genes Dev. 12:2488-2498, 1998; H. A. Tissenbaum and G. Ruvkun, Genetics 148:703,1998), and through the selection of stress-resistant mutants in either organism (T. E. Johnson, Science 249:908, 1990; S. Murakami and T. E. Johnson, Genetics 143:1207, 1996; Y. J. Lin, et al., Science 282:943, 1998). In mammals, there has been limited success in the identification of genes that control aging rates. Mutations in the Werner Syndrome locus (WRN) accelerate the onset of a subset of aging-related pathology in humans, but the role of the WRN gene product in the modulation of normal aging is unknown (C. E. Yu, et al., Science 272:258, 1996; D. B. Lombard and L. Guanrente, Trends Genet. 12:283, 1996).
In contrast to the current lack of genetic interventions to retard the aging process in mammals, caloric restriction (CR) appears to slow the intrinsic rate of aging (R. Weindruch and R. L. Walford, The Retardation of Aging and Disease by Dietary Restriction (C C. Thomas, Springfield, Ill., 1988; L. Fishbein, Ed., Biological Effects of Dietary Restriction (Springer-Verlag, N.Y., 1991; B. P. Yu, Ed., Modulation of Aging Processes by Dietary Restriction (CRC Press, Boca Raton, Fla. 1994). Most studies have involved laboratory rodents which, when subjected to a long-term, 25-50% reduction in calorie intake without essential nutrient deficiency, display delayed onset of age-associated pathological and physiological changes and extension of maximum lifespan.
The present invention will allow the evaluation of aging interventions on a molecular and tissue-specific basis through the identification of aging biomarkers. In particular, the use of gene expression profiles allows the measurement of aging rates of target organs, tissues and cells, and to what extent aging is delayed by specific interventions, as determined by quantitative analysis of mRNA abundance. Because aging-related gene expression profiles can be classified in subgroups according to function, the invention also allows for the determination of how function-specific aspects of aging are affected. This particular feature will allow for determination of combination therapies that prevent or reverse most aging related changes in particular organs, tissues, and cells.
In one embodiment, the present invention is a method of measuring the biological age of a multicellular organism comprising the steps of (a) obtaining a sample of nucleic acid isolated from the organism""s organ, tissue or cell, wherein the nucleic acid is RNA or a cDNA copy of RNA and (b) determining the expression pattern of a panel of sequences within the nucleic acid that have been predetermined to either increase or decrease in response to biological aging of the organ, tissue or cell. Preferably, the expression patterns of at least ten sequences are determined in step (b) and the organism is a mammal, most preferably a rodent.
In one preferred embodiment of the method described above, the nucleic acid is isolated from a mammalian tissue selected from the group consisting of brain tissue, heart tissue, muscle tissue, skin, liver tissue, blood, skeletal muscle, lymphocytes and mucosa.
In another embodiment the present invention is a method of obtaining biomarkers of aging comprising the steps of: (a) comparing a gene expression profile of a young multicellular organism subject""s organ, tissue or cells; a gene expression profile from a chronologically aged (and therefore biologically aged) subject""s organ, tissue or cell; and a gene expression profile from a chronologically aged but biologically younger subject""s organ, tissue or cell, and (b) identifying gene expression alterations that are observed when comparing the young subjects and the chronologically aged subjects and are not observed or reduced in magnitude when comparing the young subjects and chronologically aged and biologically younger subjects. Preferably, one uses high density oligonucleotide arrays comprising at least 5-10% of the subject""s gene expression product to compare the subject""s gene expression profile, and caloric restriction to obtain a chronologically aged but biologically younger subject.
In a preferred embodiment of the method described above, the gene expression profile indicates a two-fold or greater increase or decrease in the expression of certain genes in biologically aged subjects. In a more preferred embodiment of the present invention, the gene expression profile indicates a three-fold or greater or, most preferably three-fold or greater, increase or decrease in the expression of certain genes in aged subjects.
In another embodiment, the present invention is a method of measuring biological age of muscle tissue comprising the step of quantifying the mRNA abundance of a panel of biomarkers selected from the group consisting of markers described in the Tables 1, 2, 15 and 16. A method of measuring biological age of brain tissue comprising the step of quantifying the mRNA abundance of a panel of biomarkers selected from the group consisting of markers described in Tables 5, 6, 9, 10, 11, 12, 13 and 14.
In another embodiment, the present invention is a method for screening a compound for the ability to inhibit or retard the aging process in a multicellular organism tissue, organ or cell, preferably mammalian tissue, organ or cell, comprising the steps of: (a) dividing test organisms into first and second samples; (b) administering a test compound to the organisms of the first sample; (c) analyzing tissues, organisms and cells of the first and second samples for the level of expression of a panel of sequences that have been predetermined to either increase or decrease in response to biological aging of the tissue, (d) comparing the analysis of the first and second samples and identifying test compounds that modify the expression of the sequences of step (c) in the first sample such that the expression pattern is indicative of tissue that has an inhibited or retarded biological age.
It is an object of the present invention to evaluate or screen compounds for the ability to inhibit or retard the aging process.
It is also an object of the present invention to measure the biological age of a multicellular organism, such as a mammal in a tissue or cell-specific basis.
It is also an object of the present invention to obtain biomarkers of aging.
Other objects, features and advantage of the present invention will become apparent to one of skill in the art after review of the specification and claims.
One of the major impediments to the development of pharmaceutical, genetic or nutritional interventions aimed at retarding the aging process is the lack of a molecular method for measuring the aging process in humans or experimental animals. A suitable biomarker of the aging process should reflect biological age (physiological condition) as opposed to chronological age. Additionally, the biomarker should be amenable to quantitation, and reflect aging-related alterations at the molecular level in the tissue under study. Importantly, any such biomarker must be validated with the use of a model of retarded aging.
Caloric restriction, when started either early in life or in middle-age, represents the only established paradigm of aging retardation in mammals. (R. Weindruch and R. L. Walford, xe2x80x9cThe Retardation of Aging and Disease by Dietary Restrictionxe2x80x9d (C. C. Thomas, Springfield, Ill., 1988)) The effects of caloric restriction on age-related parameters are broad: caloric restriction increases mean and maximum lifespan, reduces and delays both spontaneous and induced carcinogenesis, almost completely suppresses autoimmunity associated with aging, and reduces the incidence of several age-induced diseases. (R. Weindruch and R. L. Walford, supra, 1988) Therefore, we expect that the rate of change of most proposed aging biomarkers should be retarded by caloric restriction.
By xe2x80x9cbiological agexe2x80x9d we mean the physiological state of an animal or tissue relative to the physiological changes that occur throughout the animal""s lifespan. By xe2x80x9cchronological agexe2x80x9d we mean the age of an animal as measured by a time scale such as month or years.
Because gene expression patterns are responsive to both intracellular and extracellular events, we reasoned that simultaneous monitoring of thousands of genes on a tissue-specific or organ-specific basis would reveal a set of genes that are altered in expression levels as a consequence of biological aging. Although alterations in gene expression with aging had been previously investigated for some genes, a global analysis of gene expression patterns during aging, and the validation of such patterns as a tool to measure biological age through the use of a model of retarded aging had not been previously performed. Such global analysis is required to identify genes that are expressed differentially as a consequence of aging on different cell types that compose the tissue under study and will allow a quantitative assessment of aging rates.
There exists a large and growing segment of the population in developed countries that is suffering from age-associated disorders, such as sarcopenia (loss of muscle mass), neurodegenerative conditions, and cardiac disease. Therefore, the market for compounds that prevent aging-associated disorders and improve quality of life for the elderly is likely to drive research and development of novel drugs by the pharmaceutical industry. As an example, many drugs, nutraceuticals and vitamins are thought to influence aging favorably, but their use remains limited due to the lack of FDA approval. The inability to assess biological aging in tissues at the molecular level precludes proper animal and human testing of such compounds.
In one embodiment, the invention is a method for measuring the biological aging process of a multicellular organism, such as a mammal, at the organ, tissue or cellular level through the characterization of the organism""s gene expression patterns. This method preferably comprises obtaining a cDNA copy of the organism""s RNA and determining the expression pattern of a panel of particular sequences (preferably at least 5 sequences, most preferably at least 10 sequences and more preferably at least 20, 30, 40, or 50 sequences) within the cDNA that have been predetermined to either increase or decrease in response to biological aging of the organ, tissue or cell. (We refer to nucleotide sequences with alterartions in expression patterns characteristic of biological age as xe2x80x9cbiomarkers.xe2x80x9d) One may characterize the biological age of the organism by determining how many and at what level the biomarkers are altered.
Tables 1-4 and 15-16 describe a specific gene expression profiles determined in skeletal muscle of mice. Tables 1, 2,15 and 16 describe aging-related increases and decreases in gene expression in gastrocnemius of mice. (Tables 1 and 2 were prepared using a high density oligonucleotide array of over 6,300 genes, while Tables 15 and 16 were prepared using a high density oligonucleotide array of 19,000 genes.) Tables 3 and 4 describe caloric restriction related decreases and increases in gene expression. Tables 1 and 2 contain a column (xe2x80x9cCR reversalxe2x80x9d) describing the influence of caloric restriction on the increased or decreased expression. Tables 5-8 describe a similar analysis of the gene expression profile determined neocortex tissue of mice and Tables 9 and 10 describe a gene expression profile determined on the cerebellum tissue in mice. Tables 11-14 describe gene expression profiles determined in mouse heart. (Tables 11 and 12 were prepared with the 19,000 high density oligonucleotide chip, while Tables 13 and 14 were prepared using the less dense gene chip.) From these gene expression profiles, one may select many biomarkers.
For example, in order to either measure or determine biological age in skeletal muscle, one would select markers in Tables 1 and 2 that reflect changes in gene expression that have been shown to be either partially or completely inhibited by caloric restriction in skeletal muscle such as AA0071777, L06444, AA114576, etc. Genes that were not affected by caloric restriction (such as W84988, Table 1) may represent chronological markers or aging, and therefore are less useful for the measurement of aging rates. One may determine which genes are or are not affected by caloric restriction by examination of the xe2x80x9cCR reversalxe2x80x9d lane of Tables 1 or 2.
If one wished to examine a tissue, organ or cell that is not represented in Tables 1-16, one would prepare samples and tabulate results from those samples as described below in the Examples. In this manner, one may examine any tissue, organ or cell for biological aging. Preferably, one would wish to examine a tissue selected from the group consisting of brain tissue, heart tissue, muscle tissue, skin, liver tissue, blood, lymphocytes, skeletal tissue and mucosa.
For example, choosing markers from Tables 1 and 2 to examine the efficacy of a test compound in aging prevention, one could design a PCR-based amplification strategy or a DNA microarray hybridization strategy to quantify the mRNA abundance for markers W08057, AA114576, 11071777, 11106112, D29016 and M16465 as a function of aging, using animals of several age groups, such as 6 months, 12 months, 18 months, 24 months and 30 months. (The marker designations refer to Gene Bank accession number entries.) A second set of animals would be given a test compound intended to slow the aging process at 10 months of age (middle age). Animals from the experimental group would be sacrificed or biopsied at the ages of 12 months, 18 months, 24 months and 30 months. If the test compound is successful, the normal aging-related alterations in expression of these particular markers will be prevented or attenuated.
One would follow the same protocol in using the other tables for marker selection. One would match the tissue to be analyzed with the appropriate table. For example, if one were analyzing muscle tissue, one might choose markers from Tables 1 and 2.
In another embodiment, the present invention is a method of obtaining and validating novel mammalian biomarkers of aging. Preferably, this method comprises the steps of comparing the gene expression profile from a young subject""s organ, tissue or cells with samples from individuals that are both chronologically and biologically aged. This is followed by comparison of the gene expression profile of the chronologically and biologically aged individuals with that of individuals that display similar chronological ages, but a younger biological age, such as animals under caloric restriction. Gene expression alterations that are prevented or retarded by caloric restriction represent markers of biological age, as opposed to chronological age.
In one version of this embodiment, one would preferably use high density oligonucleotide arrays representing at least 5-10% of the subject""s genes, as described in Lee, et al. at Science 285(5432):1390-1393, 1999 and Lee, et al., Nat. Genet. 25(3):294-297, 2000. (Both Lee, et al., supra, 1999 and Lee, et al., supra, 2000 are incorporated by reference as if fully set forth herein.)
For example, Lee, et al., supra, 1999 details the comparison between gastrocnemius muscle from 5 month (young) and 30 month (aged) mice, and 30 month mice under caloric restriction. Lee, et al., supra, 1999 disclose that of the 6500 genes surveyed in the oligonucleotide array, 58 (0.9%) displayed a greater than 2-fold increase in expression levels as a function of age and 55 (0.8%) displayed a greater than 2-fold decrease in expression. The most substantial expression change was for the mitochondrial sarcomeric creatine kinase (Mi-CK) gene (3.8-fold). Sequences that display a greater than three-fold alteration (increase or decrease) with aging, which are prevented or restricted by caloric restriction, such as W08057, AA114576, AA071777, AA106112, D29016, M16465, are likely to be particularly good aging biomarkers.
Lee, et al., supra, 2000 describes the comparison between cDNAs isolated from neocortex tissue for the same three groups of mice described above. Lee, et al., supra, 2000 disclose that of the 6347 genes surveyed, 63 (1%) displayed a greater than 1.7-fold increase in expression levels with aging in the neocortex, whereas 63 genes (1%) displayed a greater than 2.1-fold increase in expression in the cerebellum. Functional classes were assigned and regulatory mechanisms inferred for specific sets of alterations (see Tables 5-10). Of these, 20% (13/63), and 33% (17-51) could be assigned to an inflammatory response in the neocortex and cerebullum, respectively. Transcriptional alterations of several genes in this category were shared by the two brain regions, although fold-changes tended to be higher in the cerebellum, perhaps due to reduced tissue size and/or reduced heterogeneity at the cellular level. These transcriptional alterations include the microglial and macrophage migration factor Mps1 and the Cd40L receptor, which is a mediator of the microglial activation pathway. Also induced was Lysozyme C and beta(2) microglobulin which are markers of inflammation in the human CNS. Interestingly, a concerted induction of the complement cascade components C4, C1qA, C1qB and C1qC was observed, a part of the humoral immune system involved in inflammation and cytolysis.
In another embodiment, the present invention is a method of screening a test compound for the ability to inhibit or retard the aging process in mammalian tissue. In a typical example of this embodiment, one would first treat a test mammal with a test compound and then analyze a representative tissue of the mammal for the level of expression of a panel of biomarkers. Preferably, the tissue is selected from the group consisting of brain tissue, heart tissue, muscle tissue, blood, skeletal muscle, mucosa, skin and liver tissue. One then compares the analysis of the tissue with a control, untreated mammal and identifies test compounds that are capable of modifying the expression of the biomarker sequences in the mammalian samples such that the expression is indicative of tissue that has an inhibited or retarded biological age. This expression pattern would be more similar to an expression pattern found in biologically younger subjects.
As an example, a group of young rodents (mice) would be divided into a control and a test group. The test group would receive a test compound as a dietary supplement added to food from age 5 months to 30 months, whereas the control group would receive a standard diet during this time period. At age 30 months, several tissues would be collected from animals from each group, and a gene expression profile would be obtained. Each animal""s gene expression profile would be compared to that of a 5 month (young) animals receiving the standard diet. One would then examine if, for any of the organs investigated, the gene expression pattern of the animals receiving the test compound was more similar to that of young animals, compared to the experimental group that received a standard diet.
In another embodiment, the present invention is a method of detecting whether a test compound mimics the gene profile induced by caloric restriction. This method typically comprises the steps of exposing the mammal to a test compound and measuring the level of a panel of biomarkers. One then determines whether the expression pattern of the tissue mimics the expression pattern induced by caloric restriction.
For example, if one wished to examine skeletal muscle, the test compound would be analyzed for induction of genes observed to be induced by caloric restriction in Tables 3 and 4.